Plasma-mediated cutting of soft biological tissue in conductive liquid media with sub-microsecond pulses of high voltage is described in the patent of Palanker [U.S. Pat. No. 6,135,998]. Dissection of tissue based on explosive vaporization by short (under few microseconds) pulses of high voltage is described in the patent of Lewis et al. [U.S. Pat. No. 6,352,535]. In these applications an inlaid cylindrical electrode (i.e. a wire embedded into a thick insulator and exposed at its end) is applied to ionize, evaporate and fragment tissue in proximity of electrode using dielectric breakdown or vaporization of water induced by a high electric field. An inlaid cylindrical electrode cannot penetrate into tissue and thus can only produce shallow cuts on its surface. Due to the pulsed regime of application, this device produces a series of perforations in tissue, which often do not merge into a continuous cut. In addition, cavitation bubbles accompanying each pulse create substantial collateral damage in tissue during their growth and collapse phases [Effect of the Probe Geometry on Dynamics of Cavitation, D. Palanker, A. Vankov, and J. Miller, Laser-Tissue Interactions XIII, vol. 4617 SPIE (2002)]. The size of such a damage zone typically far exceeds the size of the electrode and the corresponding zone of initial energy deposition [Effect of the Probe Geometry on Dynamics of Cavitation, D. Palanker, A. Vankov, and J. Miller, Laser-Tissue Interactions XIII, vol. 4617 SPIE (2002)]. Reduction in pulse energy helps to reduce the mechanical damage, but also leads to decreased cutting depth.
A second mechanism of electrosurgical ablation is vaporization of tissue in the proximity of the probe by overheating a conductive medium with either a continuous radio frequency waveform or with sub-millisecond long bursts of pulses. This mechanism is universally applicable to soft and hard biological tissue ranging from membranes and retina to skin and cartilage. In such regimes wire electrodes are typically used, although the use of a device that could provide a uniform electric field along its length would be preferable.
Without considering end effects, the electric field in a conductive liquid at distance r from a cylindrical electrode with potential U and radius r0 much smaller than its length L is:E=U/(r ln(r0/L)),  (1)assuming that the return electrode is much larger and positioned at infinity. The threshold electric field required for dielectric breakdown in water is on the order of 105-106 V/cm [Jones, H. M. & Kunhardt, E. E. Development of Pulsed Dielectric Breakdown In Liquids. Journal of Physics D-Applied Physics 28, 178-188 (1995); Jones, H. M. & Kunhardt, E. E. Pulsed Dielectric Breakdown of Pressurized Water and Salt Solutions. Journal of Applied Physics 77, 795-805 (1995)]. Such a threshold electric field Eth can be achieved with electric pulses of several kV on a wire electrode with a diameter of several tens of micrometers. The threshold voltage required for ionization of a surface layer of water is:Uth=Ethr0 ln(L/r0).  (2)The corresponding threshold energy is:Fth=2πEth2r02L ln(L/r0).  (3)Evaporation of water in the proximity of an electrode begins when the temperature is elevated above 100° C. The threshold voltage required for vaporization of a surface layer is:Uth=(cρΔT/(τγ))1/2r0 ln(L/r0)  (4)where τ is a pulse duration, γ is the electrical conductivity of the liquid, ρ is the liquid density, c is the liquid heat capacity, and ΔT is the temperature change. The corresponding threshold energy is:Fth=2πcρΔTr02L ln(L/r0).  (5)
Lower threshold voltage and energy, as well as better localization of energy deposition can be achieved by decreasing the radius of electrode r0, as follows from equations 1-5. However, this approach is limited by the mechanical strength of the thin wire and its visibility. In addition, the problem of non-uniform distribution of electric field along the electrode, and particularly, enhancement at the apex remains.
This enhancement is illustrated in FIG. 1A, which shows the electric field surrounding a wire electrode. The field is stronger at the apex (i.e., at distance=0) and is weaker in its cylindrical portion. Thus ionization and vaporization on such an electrode will always begin and be dominant at locations of enhanced field strength, leading to uneven cutting and excessive damage in front of these singular points, as shown in FIG. 2.
One geometry that provides uniform enhancement of an electric field is a ring electrode shown in FIG. 3. Its field is uniform except for the points of deviation from perfectly round shape, such as where the ring electrode contacts with a holder. Fortunately, these regions of deviation can be kept away from tissue during surgery. The threshold voltage on such an electrode is set by the wire radius (equations 2 and 4) and thus is limited by the mechanical strength of the wire. For example, a thin wire is very weak and flexible and is thus inapplicable to manipulation of tissue. In addition, wires thinner than 25 microns are barely seen under a conventional surgical microscope, and this makes their use even more difficult. An additional problem with the application of thin wires is that erosion of thin wires greatly limits their lifetime.
Below we describe probe geometry and pulse waveform structures that provide solutions to these and other problems.